1. Field of the Invention
The present invention relates to a substrate transport apparatus.
2. Brief Description of Related Developments
The continuous demand by consumers for ever cheaper electronic devices has maintained pressure on manufacturers of the device to improve efficiency. Indeed, in the current market place, many of the devices, and to a much greater extent in the electronic and semiconductor components used in the devices, have become commodities. The desire of manufacturers of electronic and semiconductor device to increase efficiency manifests itself at all levels, but is of special significance in the design, construction, and operation of fabrication facilities or fabs. One unit by which to measure the efficiency of a given fab may be the throughput per unit of area (e.g. throughput per FT2). As may be realized from this unit of measure, the fab efficiency may be increased by raising the production rate per fabrication tool or work station of given size (space/footprint envelope), or reducing the size of the work station used to generate a greater work station density within the fab. Thus, numerous efforts have been made to increase production rates. A number of these efforts have involved providing transport apparatus capable of providing faster swap times (i.e. the time period involved in removing a workpiece from a processing module and replacing it with a new workpiece). Other efforts have been directed at reducing the overall footprint of the fabrication tool. As may be realized, there are constraints that operate against these efforts to improve fab efficiency that intrinsically arise from the fabrication system itself. For instance, fabricators currently favor fabrication system configurations for batch processing of 200 mm and 30 mm wafers that employ a general radial tool layout. This in turn defines this extension of the transport apparatus, to transport wafers to the different tool modules, which in turn impacts the overall footprint of the transport apparatus. As may be realized, in conventional transport apparatus, the longer the reach, the larger the footprint (e.g. longer links on transport elements are used to accommodate a longer reach). An example of a conventional transport apparatus is described in U.S. Pat. No. 6,669,434. This conventional apparatus is a double arm substrate transport unit that has a base arm and first and second forearms supported from the same end/tip of the base arm. Though providing a fairly compact overall footprint, this conventional transport apparatus suffers longer swap times due in part to the dynamic effects of mounting both forearms on one end of the base arm (e.g. increased polar moment of inertia, substrate transport speed during extension/retraction of arm at snap out does not have benefit or base arm motion). The present invention as incorporated in the exemplary embodiments overcomes the problems of the conventional transport apparatus as will be described in greater detail below.